The invention concerns a rotary angle sensor for a rotary member, for example for a motor vehicle wheel which requires balancing.
One form of rotary angle sensor for a rotary member such as a motor vehicle wheel to be balanced includes an emitter for emitting a light beam, and, operatively associated therewith, reflectors which are arranged at equal angular spacings from each other in the form of reflective surfaces on a rotatable circular path. The reflectors reflect the light emitted by the emitter. The assembly includes at least one detector for receiving the reflected light and producing corresponding signals which are evaluated in an evaluation device connected to the at least one detector, for detecting the rotary angle of the rotary member. A rotary angle sensor of that kind can be found in the article by Ram S Krishnan et al, A Miniature Surface Mount Reflective Optical Shaft Encoder, Hewlett-Packard Journal, December 1996, pages 1 through 6. In that rotary angle sensor, the reflectors, to maintain their flat reflective surfaces, are arranged in an annular configuration on one side of a disk fixedly connected to the rotary member, for example a shaft, extending around the axis of rotation of the disk and the rotary member. That design configuration entails additional apparatus expenditure and requires space for a properly operative arrangement of the reflectors with respect to the emitter and the detector or detectors, as the reflective surfaces extend in a plane in perpendicular relationship to the axis of rotation.
Detecting an angular position, speed of rotation and direction of rotation of a main shaft on a balancing machine, for example for ascertaining unbalance of a motor vehicle wheel, involves the use of incremental sender arrangements in which light sensors are disposed in opposite relationship to one or more light sources, more especially for example in the form of light emitting diodes. Arranged between the light source or sources and the sensors is a disk with holes, slots or teeth, which rotates with the main shaft of the machine, in which respect attention may be directed to U.S. Pat. No. 4,457,172. Besides the additional angle encoding disk, an additional zero mark in the form of a hole, slot or tooth as well as an associated detector are required to ascertain the absolute angular positions of the rotary member. This arrangement also involves the elements which specify the angular increments, extending in a plane which is perpendicular to the axis of rotation.
An object of the present invention is to provide a rotary angle sensor for a rotary member, which involves a reduced amount of apparatus expenditure and which takes up a smaller amount of space.
Another object of the present invention is to provide a rotary angle sensor for use in relation to a rotary member such as a motor vehicle wheel to be balanced, which affords reliable operational results in terms of accurate incrementation in respect of angular positions while involving a simplified structure.
In accordance with the principles of the present invention the foregoing and other objects are attained by a rotary angle sensor for a rotary member such as a motor vehicle wheel to be balanced, comprising an emitter for emitting a light beam and reflectors which are arranged in the form of reflective surfaces on a circle rotatable about an axis of rotation at equal angular spacings from each other, for reflecting light emitted by the emitter. A detector receives the reflected light and produces corresponding signals. An evaluation device is connected to the detector to evaluate the signals for rotary angle detection. The reflectors are arranged at the periphery of a circular-cylindrical surface of a rotational portion which is adapted to rotate with the rotary member, or at a circular-cylindrical surface of the rotary member.
Upon rotation of the above-mentioned rotational portion or the rotary member, the reflective surfaces or reflective surface portions which are arranged in succession on the periphery of the circle are passed successively through the beam which is preferably continuously emitted by the emitter. The reflective surfaces or surface portions constituting the reflectors are of such a configuration in respect of their succession that they direct discrete reflected beams on to the detector. That provides for incrementation in respect of angular positions which correspond to the respective reflective surface portions. The respective surface portions can directly adjoin each other and may have a continuous reflective band or strip with features distinguishing the individual surface portions. For that purpose the respective surface portions may be of surface configurations which differ from the shape of a circular cylinder. It is however also possible to use circular-cylindrical surfaces for the respective surface portions, wherein the radius of the respective circular-cylindrical surface can differ from the radius of the circle which has the circular-cylindrical surface on which the surface portions are disposed. The radius of the respective surface portion may be between infinite, that is to say -a flat surface, and the radius of the circular-cylindrical surface on which the reflectors are arranged. It is however also possible for the surface portions to involve a parabolic or elliptical cross-section.
The reflective surface portions may also be of a convex or concave configuration, relative to the emitter or detector.
In a preferred feature the reflective surface portions may be arranged directly in a row with each other on a flexible band, with the flexible band being wrapped around the circular-cylindrical surface of the rotor or the rotational portion.
In a preferred embodiment of the invention the reflectors are formed at surfaces of lands which extend in parallel relationship with the axis of the circular-cylindrical surface. For that purpose the lands may be arranged between two strips of material and can be integrally joined thereto to form a band. The band has flat or curved surfaces in the region of the lands and the strips. When the band is bent to apply it to the circular-cylindrical surface, the surfaces of the strips match the circular-cylindrical shape. It will be appreciated however that the lands may retain their flat surfaces. The width of the lands, that is to say the extent thereof in the peripheral direction of the circular-cylindrical surface, is a fraction of a millimeter, for example about 0.3 mm. When the band with the lands having the reflectors is wrapped around the rotary member or around the rotational portion which is adapted to rotate with the rotary member, it does not in practice require any space as this arrangement provides that the diameter of the rotary member or rotational portion is only immaterially increased by the thickness of the band.
As noted above the reflectors are at equal angular spacings from each other.
To achieve an absolute angular relationship, an angular spacing which differs from the other equal angular spacings may be provided between two adjacent reflectors. That for example provides a zero marking.
The polygonal mirror configuration which is afforded in the structure according to the invention and whose reflectors, formed by reflecting surfaces, are arranged around the periphery of the circular-cylindrical surface, can also be formed from a rigid shaped portion, for example a molded portion, such as plastic material. The reflective surface portions of the reflectors can be produced by metal deposition, in particular galvanic metal deposition.
More particularly in a preferred feature of the invention the shaped portion can be an injection molding of plastic material in a sleeve configuration. The shaped portion can form the rotational portion which is adapted to rotate with the rotary member. As the reflective surface portions which are arranged directly in succession can be distinguished from each other by the detector, it is possible to use a continuous reflective coating.
Preferably, the light beam emitted on to the respective reflectors is focused so that its focal point is on the flat surface of each respective reflector. During the rotational movement, the beam reflected by a respective reflector is diverted within the surface of the respective reflector in dependence on the changing rotational angle and in that situation can be received by a plurality of detectors which are arranged in a row.
Instead of the plurality of detectors it is also possible for a detector to comprise a plurality of light-sensitive sensors, on which the reflected beam deflected by the respective reflector successively impinges during the rotational movement. The detectors or sensors are arranged in mutually juxtaposed relationship substantially in the direction of rotation. They may also be arranged in a rectilinear row but in a preferred feature they are disposed on a circular arc around the axis of rotation. A reversal in the direction of rotation of the rotary member can be detected by the light beam being directed for a longer period of time on to the sensor or detector at which the reversal in the direction of rotation takes place. The signals outputted by the detectors or sensors are correspondingly evaluated by the evaluation device.
Further objects, features and advantages of the invention will be apparent from the description hereinafter of embodiments of the invention.